AbstractIn fall migration 2012 an avian nocturnal flight call monitoring station at Minatitlán, Veracruz, Mexico logged a previously unreported call type. The simultaneous two-toned nature of the vocalization is consistent with production from a syrinx (i.e. not from a bat or flying squirrel). The less than 100 ms duration of the call, the 6-10 kHz frequency range, and the characteristics we collected of its nightly and hourly periodicity are congruent with calls given by many small passerines that migrate at night across North America. Yet the call is distinct from night flight calls of neotropical migrant species that regularly traverse eastern USA in spring and fall migration. We have considered the known species that migrate through Veracruz and there are a number of candidates for the caller, but the identity has eluded us. We submit here the mystery and evidence we have gathered for review by others who may have special insight or who may carry on the investigation. Identifying the call would enable the possibility for automatically documenting nocturnal migration activity of the species across its range with specialized audio processors.